Aluminum alloy is widely used due to its lightness in weight and high thermal conductivity, for example, for vehicular heat exchangers. Many heat exchangers are manufactured by heating and brazing a sheet material (so-called “brazing sheet”) which has an aluminum core alloy cladded with a brazing filler alloy.
Since the brazing filler alloy of the sheet material contains aluminum as a major component, an oxide layer is easy to be formed on a surface of the brazing filler alloy during brazing of the sheet material. If the oxide layer is formed, fluidity (wettability) of the brazing filler alloy may decrease, and the brazing of the sheet material may be inhibited accordingly. Thus, a flux is generally applied to the brazing filler alloy in the brazing of the sheet material in order to remove the oxide layer from the surface of the brazing filler alloy.
However, if the flux is used in the brazing of the sheet material, a residue of the flux may be produced and may cause clogging of passages of the heat exchanger. As a measure for this, a brazing method without using a flux has been developed recently. For example, a brazing method using a sheet material, in which a core alloy or a brazing filler alloy contains Mg, is disclosed in Patent Document 1 (JP 2004-358519 A corresponding to US 2006/0081682 A1).
Mg has a property of destroying the oxide layer of the brazing filler alloy. Thus, by adding Mg to the core alloy or the brazing filler alloy, the fluidity (wettability) of the brazing filler alloy can be improved, and the sheet material can be brazed easily. The brazing is performed, for example, in an inert gas atmosphere under an oxygen partial pressure of 20 Pa approximately.
However, the property of destroying the oxide layer of Mg is generally lower than that of flux. Thus, though the oxide layer of the brazing filler alloy is removed by Mg, the surface of the brazing filler alloy may be oxidized again by a tiny amount of oxygen. In this case, the fluidity (wettability) of the brazing filler alloy may decrease due to the oxide layer formed again, and the brazing filler alloy may not spread sufficiently. As a result, a fillet of the brazing filler alloy may be not formed, or the shape of the fillet may be unstable. In other words, brazability of the sheet material may not be improved sufficiently.